1. Field of the Invention
The present invention relates to a bearing device for an optical deflection scanning device used in an information apparatus, a laser beam printer, and the like, specifically to a dynamic pressure gas bearing device incorporated, for example, in an optical deflection scanning device and used for supporting a polygon mirror which rotates at high speed.
2. Description of the Prior Art
The dynamic pressure gas bearing device maintains a rotating member and a fixed member in a non-contacting state by dynamic pressure produced from dynamic pressure generating grooves formed in one or both mutually opposing surfaces on the two members. Accordingly, there is almost no friction acting between the rotating member and the fixed member. Power used to rotate the rotating member is reduced and heat of friction produced during rotation is prevented.
FIG. 1 shows an example of such a conventional type of dynamic pressure gall bearing device used in an optical deflection scanning device. This conventional example is provided with a cylindrical hole 2 in the center of a housing 1, which is a fixed member. The housing may have a sleeve along the inner peripheral surface of the housing 1. A cylindrical radial bearing surface 3 is provided on the inner peripheral surface of the cylindrical hole 2, and a thrust bearing surface 5 is provided at the center of the bottom surface of the hole 2 with an elevated section 4 of a convex form.
A shaft 6, which is a rotating member, is fitted into the cylindrical hole 2 in the housing 1. A mating radial bearing surface 7 is provided on the outer peripheral surface of the shaft 6, opposing the radial bearing surface 3. Also, a mating thrust bearing surface 9 is provided opposing the thrust bearing surface 5 on one end (lower end in FIG. 1) of the shaft 6. Grooves 8 are provided in the radial bearing surface 7 of the shaft 6 to create dynamic pressure. Specifically, the grooves 8 comprise spiral grooves 8a for the operation of the thrust bearing and herringbone-shaped grooves 8b for the radial bearing, thereby forming a hydrodynamic fluid bearing equipped with dynamic pressure generating grooves. Air is used as the hydrodynamic fluid in this example. Specifically, the shaft 6 rotates at a high speed, so that a dynamic pressure is created between the inner peripheral surface of the housing 1 and the outer peripheral surface of the shaft 6.
When the shaft 6 is stationary, the thrust bearing surface 9 at the lower end of the shaft 6 contacts the elevated section 4 of the thrust bearing surface 5 of the housing 1 and supports an axial load. Then, as the shaft 6 is rotating, dynamic pressure resulting from this rotation is generated at the spiral grooves 8a provided on the radial bearing surface 7 and conducted between the thrust bearing surface 5 and the mating thrust bearing surface 9, so that the axial load is subjected to this pressure for supporting. Accordingly, the shaft 6 rotates without contacting the housing (sleeve) 1 during the rated operation of the bearing device.
A mirror 50 is mounted on the upper section of the shaft 6. The mirror 50 is rotatingly driven together with the shaft 6 by a drive motor 60. The drive motor 60 has a rotor magnet 61 formed from a permanent magnet and mounted on the shaft 6 through a casing 63. A stator coil 64 which opposes the rotor magnet 61 in the radial direction is mounted on the housing 1. In this conventional example, the mirror 50 and the rotor casing 63 are fitted onto the outer peripheral surface of the shaft 6. In addition, the rotor casing 63 is secured to a flange section 6a of the shaft 6 by a screw 65 to secure the mirror 50 between them. The rotor magnet 61 is also secured by an adhesive to the casing 63.
The mirror 50 is made from an aluminium alloy; the casing 63 is made from an iron alloy or steel; and the shaft 6 is made from an aluminium alloy composite containing carbon fiber. The coefficients of linear expansion of these materials are all different.
This type of optical deflection scanning is used in a laser printer or in a digital copier. To reduce the size of the entire machine it is necessary to have a short device in the axial direction. However, with a conventional example, long spiral grooves extending in the axial direction are required to support the axial load on the radial bearing surface, therefore there is the problem that the length of the device in the axial direction cannot be reduced.
In addition, for satisfactory operating characteristics, a short starting period is necessary. This requires a unit which takes a short time to attain a rated rotational speed. Specifically, in a laser printer or in a digital copier, a short time interval is desirable from throwing the switch until the device can be used. Therefore, required is an optical reflection scanning device in which the time interval is as short as possible until reaching the rotational speed at which the device can be used. However, with the conventional example, because in the thrust bearing section, the end surface of the shaft and the bottom surface of the housing (sleeve) are in contact with each other when the rotating member is stationary, the torque from the starting friction is large. In addition, a device must be long in the axial direction enough to ensure an axial load capacity as mentioned above. Accordingly, the inertia of the shaft 6 and a flange section 6a is large, so that it is difficult to reach operating speed within a short time.
In the case of the dynamic pressure gas bearing device, the rotating member and the fixed member are prevented from coming into contact by using the dynamic pressure of the gas (mainly air) as a support force. In the case of a gas, the viscosity, which has a large effect on this support force, is extremely small. Therefore when the speed of rotation of the rotating member is not as high as required, adequate support is not obtained. In other words, while the rate of rotation is low so that the support force is inadequate, the opposing surfaces of the rotating member and the fixed member rub together.
Because there is almost no lubricating capability in the gas which is the dynamic pressure fluid such as air, unless some sort of countermeasures are taken, the rubbing surfaces of the rotating member and the fixed member will quickly wear and the durability of the dynamic pressure gas bearing device is adversely affected.
For this reason, the opposing surfaces of the rotating member and the fixed member are conventionally formed from a ceramic material with superior anti-friction characteristics, or a metal material with lubrication characteristics, such as gold, TiN, or the like is plated onto these surfaces.
However, in the case where the rubbing or sliding surfaces are formed by e.g. a ceramic material, the material and processing costs are high, resulting in high production costs for the dynamic pressure gas bearing device. Also, when the surfaces are plated, not only are the production costs high due to machining steps required before and after plating but sufficient lubrication is not always obtained. When a comparatively large load is added to the plated rotating member, there are cases where adequate durability is not obtained when the rotating member is repeatedly stopped and started.
In addition, higher and higher rates of rotation are demanded as improvements in the recording density and increases in printing speed progress. Also, vibrations produced by the optical deflection scanning device during rotation must be restrained to an extremely low level to provide an improvement in printing quality.
When such a rate of rotation is obtained, it becomes increasingly important to maintain imbalance as less as possible because the centrifugal force resulting from the imbalance in the rotating section increases in proportion to the square of the velocity. However, when the rate of rotation becomes high, a large amount of heat is developped because of windage loss from the mirror 50 and the friction in the bearing section in the example of FIG. 1. Therefore, no matter how precise an imbalance modification (balance correction) is applied at normal temperatures, the difference in the coefficients of linear expansion of the various materials resulting from a temperature increase causes the centers of gravity of the mirror 50 and the rotor magnet 61 to slightly move with respect to the center of rotation of the housing (sleeve) 1 in the example of FIG. 1. There is therefore the problem that the vibration of the device increases with time. In addition, even if balance correction is applied at a higher temperature in advance, the vibration increases when rotation next commences. Or, it is almost impossible to obtain enough reduction of vibration under normal operation because the center of gravity is inclined to change irregularly as temperature increases.
These problems in the conventional example of FIG. 1 are caused by the facts; very small gaps are present in the fitting surfaces between the mirror 50 and the shaft 6 and between the casing 63 of the rotor magnet and the shaft 6, and materials with different coefficients of linear expansion are utilized for the mirror 50, the casing 63 of the rotor magnet, and the flange 6a. In addition, because an adhesive is used for bonding the rotor magnet 61 and the casing 63, the gap between the rotor magnet 61 and the casing 63 is filled with the adhesive, so that, although only a very slight thermal expansion occurs, this causes the phenomenon of irregular movement in the gap and the adhesive layer. It should be noted that the adhesive between the rotor magnet 61 and the casing 63 is subjected to elastic deformation from thermal expansion of the rotor magnet 61 and casing 63.
As a result of recent improvements in printing quality, the development of the low vibration device has become increasingly necessary. The effect of the vibration caused by the imbalance due to the development of heat cannot be ignored in this type of high speed rotation, although conventionally this has not been that much of a problem.